These days, wheels with magnetic bearings intended for space applications in gyroscopic actuators comprise a rotor in levitation about a stator. This levitation is controlled by an electronic servo-control based on a position sensor, an actuator, conventionally a magnetic bearing, making it possible to provoke displacements of the rotor, and an electronic device which controls the actuator according to information given by the position sensor.
The position sensor concerned consists of a set of inductive sensors. It generally comprises a detection ring situated on the stator and a detection track situated on the rotor. The detection track is produced in a conductive material, generally a magnetic. The position sensor makes it possible to know the distance between the detection ring and the detection track.
However, the detection track, generally cylindrical, has defects which may be surface defects, or defects associated with the material itself: resistivity, particle size, etc. These defects cause spurious microvibrations of the rotor, because they are “seen” by the position sensor, and the electronic servo-control leads to multiple rotor displacement commands.
By definition, the defects of the detection track are periodic, since, on each revolution, a defect of the detection track passes once again in front of the same sensor. They can therefore be broken down into Fourier series comprising a fundamental component and harmonics of order 1 to N. The fundamental component is not a property of the detection track but rather a mechanical property of the complete magnetic bearing wheel device. It can be filtered in various ways.
The present invention applies to a context in which it is important to minimize the spurious microvibrations of the rotor.
The solutions that are currently envisaged fall into three categories: improving the detection track in order to minimize its defects; improving the sensor so that it does not “see” the defects of the detection track; filtering the defects using the electronics.
First of all, it is known by coupling two diametrically opposing sensors, the even order harmonics of the spurious vibrations can be eliminated. The position sensors of the state of the art can, for example, comprise four sensors coupled in pairs.
Furthermore, for the sensors to “see” a minimum of defects of the detection track, their angular aperture must be increased: thus, the state of the art has moved on from one-off sensors to sensors having an angular aperture close to 90°. These spread sensors “see” fewer defects, because they measure average values over a wide angle, which makes it possible to eliminate the high-order harmonics. However, too many spurious micro-vibrations remain; they correspond to the low-order harmonics of the Fourier series breakdown of the defects of the detection track.